The term “polymers”, as generally understood in the art, are molecules composed of repeating structural units, or monomers, connected by covalent chemical bonds. Well known examples of polymers include plastics, DNA and proteins.
As a class of material, polymers are versatile and has played an important part in modern industrialized society. The range of physical and chemical properties that may be obtained by putting together different monomers to form different polymers appears to show endless possibilities. One method of achieving this structural diversity is by stringing together different types of monomers to form a polymer molecule. A polymer molecule derived from two or more monomeric species is referred to as a copolymer, as opposed to a homopolymer (Polymer 1 of FIG. 1), which is composed purely of one single monomeric species.
Accordingly, the term “copolymerization” refers to methods used to chemically synthesize a copolymer. Commercially relevant copolymers include ABS plastic, SBR, styrene-isoprene-styrene (SIS) and ethylene-vinyl acetate (EVA).
Since a copolymer consists of at least two types of constitutional units (not structure units), copolymers can be classified based on how these units are arranged along the chain. These include: alternating copolymers (Polymer 2 of FIG. 1) with regular alternating A and B units; periodic copolymers with A and B arranged in a repeating sequence (e.g. A-B-A-B-B-A-A-A-A-B-B-B)n; random copolymers (Polymer 3 of FIG. 1) with random sequences of monomer A and B; statistical copolymers in which the ordering of the distinct monomers within the polymer sequence obey known statistical rules; and block copolymers (Polymer 4 of FIG. 1) which comprises two or more homopolymer subunits linked by covalent bonds. The union of the homopolymer subunits may require an intermediate non-repeating subunit, known as a junction block. Block copolymers with two or three distinct blocks are called diblock copolymers and triblock copolymers, respectively.
Copolymers may also be described in terms of the existence of or arrangement of branches in the polymer structure. For example, linear copolymers consist of a single main chain whereas branched copolymers consist of a single main chain with one or more polymeric side chains.
Graft copolymers are a special type of branched copolymer in which the side chains are structurally distinct from the main chain. Polymer 5 of FIG. 1 depicts a special case where the main chain and side chains are composed of distinct homopolymers. However, the individual chains of a graft copolymer may be homopolymers or copolymers. Note that different copolymer sequencing is sufficient to define a structural difference, thus an A-B diblock copolymer with A-B alternating copolymer side chains is properly called a graft copolymer.
Other special types of branched copolymers include star copolymers, brush copolymers, and comb copolymers.
In man-made plastics, copolymerization is used to modify the properties of the material to a specific needs, for example to reduce crystallinity, modify glass transition temperature or to improve solubility. In this regard, block copolymers are of particular interest because they can “microphase separate” to form periodic nanostructures.
Microphase separation is a situation similar to that of oil and water. Oil and water don't mix together-they macrophase separate. Given a first “oil-like” block and a second “water-like” block, the block copolymers will undergo microphase separation because the blocks will want to get as far from each other as possible. But they are covalently bonded, so they are not able to get very far, hence, they can only “microphase separate”. Under such conditions, the “oil” and “water” or hydrophobic and hydrophilic blocks tend to form nanometer-sized structures. These structures can look like spheres of polymethyl methacrylate (PMMA) in a matrix of polystyrene (PS) or vice versa, or they could be stripes (often called laminates) or cylinders. The nanostructures created from block copolymers could potentially be used for creating devices for use in computer memory, nanoscale-templating and nanoscale separations.
As explained above, the versatility of polymers is due in large part to its structurally variety. Blending homopolymers, as opposed to copolymerization, is believed to be another effective method for obtaining polymer materials with a desirable combination of the properties from two or more components. However, most polymer pairs are immiscible and are referred to as being “incompatible”. Thus, two separate phases are formed. Alternatively partial mixing occurs leading to relatively large sized separate domains (>1 micron) corresponding to each of the components. This typically leads to poor mechanical and optical properties that often have the worst rather than best combination of properties.
In contrast, so called AB or ABA “block copolymers” in which the A and B polymer blocks are covalently attached tend to give much smaller lamellar or other highly organized domains (order of 20-100 nm) that have superior properties. Examples are the so called “Kratons” thermoplastic elastomers (rubbers) where two outer “hard” PS blocks enclose the inner elastomeric block consisting of polyisoprene (PI) or polybutadiene (PBD). In the solid state at room temperature such elastomers are “crosslinked” through the hard PS domains. Upon heating well above the so called glass transition (softening) temperature of polystyrene (about 100° C.) these domains lose their cohesive strength and the PS-PI-PS or PS-PBD-PS polymers now become liquid-like and, unlike conventional elastomers, they may be reprocessed. However, the synthesis of such block copolymers typically is not trivial, and it is expensive. Thus, the block copolymerization approach has not yet found wide industrial applications and blending is still preferred.
Another conceivable approach to overcome the shortcomings of blending is to chemically modify the homopolymers to introduce attractive interactions that stabilize the interfaces between the incompatible blend components. Examples are hydrogen bonding, acid-base, charge-transfer, ion-dipole, donor-acceptor adducts and transition metal complexes. However, such chemical modifications typically are impractical and/or expensive.
Therefore, there still exists a need for better methods of blending “incompatible” polymers to form novel polymeric materials.